Prior to setting forth a short discussion of the related art, it may be helpful to set forth definitions of certain terms that will be used herein.
The term “cell” as used herein, in the context of cellular communication, is a land area served by at least one fixed-location transceiver, known as a cell site or base station. In a cellular network, each cell may use a different set of frequencies than that used by neighboring cells for cell-edge UEs, to avoid inter-cell interference becoming the limiting factor of system throughput.
The term multiple input multiple output or “MIMO” as used herein, is defined as the use of multiple antennas at both the transmitter and receiver to improve communication performance. MIMO offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. It achieves this goal by spreading the transmit power over the antennas to achieve spatial multiplexing that improves the spectral efficiency (more bits per second per Hz of bandwidth) or to achieve a diversity gain that improves the link reliability (reduced fading), or increased antenna directivity.
The term “beamforming” sometimes referred to as “spatial filtering” as used herein, is a signal processing technique used in antenna arrays for directional signal transmission or reception. This is achieved by combining elements in the array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity.
The term “beamformer” as used herein refers to RF circuitry that implements beamforming and usually includes a combiner and may further include switches, controllable phase shifters, and in some cases amplifiers and/or attenuators.
The term “Receiving Radio Distribution Network” or “Rx RDN” or simply “RDN” as used herein is defined as a group of beamformers as set forth above.
The term “hybrid MIMO RDN” as used herein is defined as a MIMO system that employs two or more antennas per channel (N is the number of channels and M is the total number of antennas and M>N). This architecture employs a beamformer for each channel so that two or more antennas are combined for each radio circuit that is connected to each one of the channels.
Given a receiver which is equipped with more antennas than baseband supported receivers, these extra antennas are used to form an RF beamformer for each one of the receiver's inputs. When the phases of the received signals from each antenna connected to a receiver input are properly adjusted with respect to one another, the individual signals may be passively combined and result in an improved Signal to Interference plus Noise Ratio (SINR) for the receiving system.
Channel estimation can be used for adjusting the phases of the received signals in order to maximize the received power of a desired signal. However, channel estimation works well only when external noise is uncorrelated, and therefore may not be optimal for cases of correlated external noise. Specifically, at interference limited situations, further SINR improvement is possible when such interference is taken into account.